Surface reflectance spectroscopy studies of chemisorption on W(100)

Abstract

Surface reflectance spectroscopy (SRS) has been used to study chemisorption-induced changes in the electronic structure of a clean W(100) surface. The relative changes in optical reflectance, RR, caused by chemisorption of H2 CO, and O2 have been measured as a function of exposure and photon energy in the range 0.6<<4.8 eV for adatom coverages up to 1 monolayer. Structure in the exposure and coverage dependence of RR displays the various adatom binding states and also indicates the presence of adatom-adatom interactions within a given binding state. The model of McIntyre and Aspnes (MA) is adopted to relate the observed RR() to changes in an effective complex surface dielectric function ^s(); by assuming a simple form for ^s(), we fit RR() spectra calculated using the MA model to the experimental data and thereby obtain difference spectra ^s() which show the chemisorption-induced changes in the optical response of the surface region. The positions of prominent peaks and dips in 2s(=Im^s) give the energies of important optical transitions associated with the surface electronic structure which are produced or quenched by chemisorption. These energies show a strong correlation with the position of surface levels (relative to the Fermi energy EF) seen in ultraviolet photoemission and field-emission spectroscopies. As a result, the excitations obtained from SRS are attributed to optical transitions from intrinsic surface states and adsorbate-induced surface orbitals to final states concentrated primarily at EF, which are most likely the tails of extended (Bloch) states of the metal as modified at the surface. © 1974 The American Physical Society.